In a nonlinear optical microscope, such as a stimulated Raman scattering microscope, configured to exploit a nonlinear optical process, pulsed lights emitted by two pulsed lasers need to be focused on a sample with timings of their pulses being synchronized (or a difference between the timings being kept constant).
PLT 1 discloses a stimulated Raman scattering (SRS) microscope that detects, as a pulse timing difference, an output from a photodetector that detects two-photon absorption and adjusts a pulse period so that the detected value is equal to a set value. PLT 2 discloses a coherent anti-Stokes Raman scattering (CARS) microscope that adjusts a pulse period based on a difference between outputs from two photodetectors used to detect a pulse timing difference.